They are definitely still being produced in their millions and the most recent 'Era' is Sun & Moon and at present the 6th set in this series is called 'Forbidden Light' and the 7th set ('Celestial Storm') will be released in the UK in August. As for how many sets there are.... several hundred if you include theme decks, promo cards, Japanese variants etc. It is HUGE and is collected avidly across the globe with America, Japan and the UK being the biggest markets.

The sets are issued in several languages though too and you can find Russian (certainly for the XY era), Portuguese, Korean, French, German etc to add a little more variation into the mix

I'd always recommend Bulbapedia as the one stop shop for info on the different series.

Just a note on that for you David. The Topps series aren't especially sought after; it's the blue backed 'official' TCG cards that you should look to target. Condition with Pok?mon is now heavily dominated by the PSA too. As much as they like to focus on sport I think a growing trend has been that they're seeing as many Pok?mon submissions as baseball (if not more). As a result you need to look at the back of the cards to check for little white marks along the blue edge as these are usually what will dictate the market price.